MRI magnets for whole-body imaging require to achieve high homogeneous magnetic field in a large space, such as a magnetic field non-homogeneity less than 10 ppm (parts per million) in a range of 40 cm diameter spherical volume (DSV). Generally, magnet structures are in form of tunnel, and can provide high field and high homogeneity, as is the case with most of the present magnet structures. However, magnet structures in form of tunnel have unsatisfied openness, and may cause cabin fever for some patients. Although many new structural magnet systems have emerged in recent years, such as a short cavity magnet structure with magnet lengths from 1.3 m to 1.4 m, it is still difficult to meet the need for openness in interventional therapy.
From the point of the development of interventional therapy and medical diagnosis technology, there is a need for fully-open magnet systems to accommodate the need in medical interventional therapy. Current fully-open MRI products focus on permanent magnets, which may provide central magnetic field below 0.7 T, and generally adopt “C” shape structures. WO/2007/094844 provides an open MRI permanent magnet structure, which may have a central magnetic field of up to 1 T. WO/1998/007362 provides a double-side structure MRI permanent magnet. Chinese patent 02210965 provides a two-column C type open magnetic resonance permanent magnet.
A few companies have also developed open magnetic resonance superconducting magnets with field intensities generally below 1.2 T, such as related products that are available from Hitachi Company and Philips Company. China patent 02824552 by Philips Company provides an open magnet structure with a pair of superconducting coils. At present, there are no 1.5 T fully-open MRI systems available in the world.
The main difficulty in open MRI magnets with superconducting coils lies in high cost and difficulties in fabrication technology. For open superconducting magnet structures with passive shield, the magnet system will become too big due to the incorporation of ferromagnetic shield. Moreover, open magnets with active shield have larger ratios of maximum magnetic field to central magnetic field. For example, in the case of a 1.5 T central magnetic field, the maximum magnetic field within the coils may even exceed 10 T, which is not acceptable for superconducting coils adopting NbTi as their material. Thus, there is a need to invent a new magnet structure to overcome this problem.